I have a memory of an Iron Horse House one but that might just be the Mandela Effect. Maybe check UKTS? Somebody might have made a low-quality one years ago.
I never saw one on the IHH site or UKTS. Considering the number of third rail routes in the South East it is surprising a Class 71 has not been made. Then it would be an easier step to a Class 74 that some of the Class 71s were rebuilt from.